In the 1960s, many trials for the preparation of an organic electroluminescence device were reported by using conjugated materials generally having a fused aromatic ring (U.S. Pat. No. 3,172,862, issued 1965; U.S. Pat. No. 3,173,050, issued 1965). The efficiencies and lifetimes of these organic EL devices were much lower than those obtained from inorganic systems at the same time, so research mainly focused on the inorganic materials. The reason for the low luminance of the early organic EL device is the highly resistive EL medium, which prevents the efficient injection of carriers into the light-emitting layer. Tang and VanSlyke solved this problem successfully in the late 1980s (Tang and VanSlyke, Appl. Phys. Lett. 1987, 51, 913). They improved the performance of an organic EL device significantly by using a structure made of two thin layers: a hole transporting layer of an organic substance laminated on an organic emitting layer. This work revived the research on organic EL devices, and resulted in the development of a new generation of light-emitting diodes with organic dyes. Since then, much work has been done to further improve the efficiency, stability, color purity and so forth of such a device. (U.S. Pat. Nos. 5,141,671; 4,539,507; 6,020,078; 5,935,720; 5,972,247; 5,593,788; 4,885,211; 5,059,862; 5,104,740; 5,069,975; 5,126,214; 5,389,444; 6,165,383; 6,245,449; Chen, Shi and Tang, Macromol. Symp., 1997, 125, 1; Segura, Acta. Polym., 1998, 49, 319; Mitschke and Bauerle, J. Mater. Chem. 2000, 10, 1471). Among these, one of the most convenient and useful methods is to dope a strong emitting material into a host material to form a guest-host system. Thus, in principle, an organic EL device with good efficiency and high stability, as well as desired color with proper chromaticity, can be obtained by doping different strongly emitting materials into a host material, such as tri-(8-hydroxyquinolinato)aluminum (AlQ3), to meet the requirement of the practical applications. As a general rule, the energy gap between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of a host material should be larger than that of the doped guest material to allow an efficient energy transfer from the host to guest.
AlQ3 is one of the most useful host materials in organic EL devices. It has been used as the host material for a green-emitting device (Shi and Tang, U.S. Pat. No. 5,593,788), as well as a red-emitting device made of 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) derivatives (Chen, Tang and Shi, U.S. Pat. No. 5,935,720). As for the guest materials that emit at longer wavelengths, such as green, yellow and red regions, two kinds of compounds are potential candidates: (i) compounds with large fused homo-aromatic rings; and (ii) intramolecular charge transfer (ICT) compounds with electron donating (D) groups and electron withdrawing groups (A) linked by conjugated structure. Since the fused homo-aromatic compounds with large conjugated structures, especially those that emit in the red, are often oxidized easily by singlet oxygen at ambient conditions, these compounds are not expected to be suitable dopants unless the devices are used in the dark or without oxygen. Compared with the former compounds, ICT compounds have the following advantages:                (i) The emission wavelength can be easily tuned by changing substituents to get different colors;        (ii) The molecular structure is relatively easy to modify for desired properties;        (iii) Their Stokes shifts are generally large to prevent efficient self-re-absorption; especially in the solid state; and        (iv) They are chemically stable, not easily oxidized by singlet oxygen.        
Pyrazolo[3,4b]quinoxaline derivatives have been used as photo-initiators (Kucybala et al., J. Photochem. Photobiol., A, 2000, 136(3), 227; Perkin 2, 2000, 7, 1559) and as antibacterial agents. The preparation of some 7-dimethylamino, diethylamino substituted pyrazolo[3,4b]quinoxaline derivatives was reported in the 1980s (Mann et al., J. Prakt. Chem., 1986, 328(3), 342). On the other hand, these materials have not been used in organic EL devices. The pyrazolo[3,4b]quinoxaline derivatives in the present invention are typical ICT compounds, so these compounds are expected to have the general features of ICT compounds mentioned above. The compounds disclosed herein exhibit a very high fluorescence and a narrow emission band, as indicated in FIG. 2. Thus, it is desirable to use these pyrazolo[3,4b]quinoxaline derivatives as emitting materials to achieve organic electroluminescence devices with improved efficiency and color purity.